Gallium Nitride (GaN) is a wide-gap semiconductor that is used today for optoelectronic and electronic devices, such as visible light-emitting diodes and lasers, and high-power microwave electronic transistors. Such devices produce a high amount of heat requiring a heat spreader to be attached to GaN layers to ensure that electrical properties of the semiconductor remain within a safe temperature operating region. One of the known approaches is to use a GaN layer disposed on a SiC substrate configured for thermal management.
For example, U.S. Pat. No. 9,111,750 to Kashyap et al. describes a monolithically integrated semiconductor assembly. The semiconductor assembly includes a substrate including silicon carbide (SiC), and a gallium nitride (GaN) semiconductor device is fabricated on the substrate.
However, although this approach has been shown to be a viable solution for fabrication of GaN microwave devices, it still cannot overcome heat loads, and suffers from rather insufficient thermal conductivity of the SiC substrate. Accordingly, to further enhance thermal management, a logical approach was to replace the SiC substrate, having a thermal conductivity around 350-400 W/mK, with the highest thermal conductivity material available, such as diamond. Although natural diamond is an excellent thermal conductor, this material cannot be widely used for electronic applications due to its scarcity and cost.
Integration of a synthetic polycrystalline diamond, that can have a thermal conductivity in the range of 800-2000 W/mK, deposited by chemical-vapor deposition (CVD) into semiconductor processing techniques, has evolved over the past several years. A polycrystalline diamond means that it consists of diamond crystals variously oriented or composed of more than one crystal.
For example, GaN-on-diamond technology and resulting devices involving structures which feature atomically attached GaN epilayers to synthetic diamond substrates are described in U.S. Pat. Nos. 7,595,507 and 9,359,693 to Fransis et al. This technology enables bringing together the best heat conductor (i.e., diamond) with electronic and optoelectronic devices based on GaN-related compounds.
However, integration of a CVD diamond introduces many challenges due to interface issues associated with material lattice mismatch and with different thermal expansion of the GaN and diamond layers, that can be a reason for bowing and warping of the GaN-on-diamond structure and its delamination. Moreover, diamond-based composite substrates require careful attention to thermal resistance of the interface layer formed between the GaN and the diamond, which can diminish the benefits of using this high conductivity material.